A glucose-containing tetrasaccharide with the chemical structure Glc alpha 1-6Glc alpha 1-4Glc alpha 1-4Glc is excreted in abnormally large amounts in the urine of individuals with glycogenoses and in some types of muscular dystrophy (G. Lennartson et al. Biomed. Mass Spectrometry 3: 51-54, 1976). This oligosaccharide probably represents an abnormal metabolite of glycogen in that it occurs as a limit dextrin after exhaustive treatment of glycogen with amylase. Using an immunogen prepared by coupling the oligosaccharide to the protein carrier KLH we have obtained specific antibodies that can be used to detect 100 picomoles of the reduced oligosaccharide by radioimmunoassay. This assay will be used to study the relationship between glycogen metabolism and excretion of the oligosaccharide.